Field of the Invention
The invention relates to an exhaust gas after-treatment device and a method for exhaust gas after-treatment.
Description of the Prior Art
The nitrogen oxide or NOx emissions limit values, which are becoming evermore stringent for diesel- or lean-engine-operated vehicles, require exhaust gas posttreatment that reduces the nitrogen oxides, beyond a certain vehicle weight. A very effective exhaust gas posttreatment known from the prior art is known as the selective catalytic reaction (SCR). In it, a reducing agent, namely ammonia, is injected as needed into the exhaust gas posttreatment device of the internal combustion engine and reacts in a special catalytic converter together with the nitrogen oxides of the exhaust gases to form the harmless compounds of nitrogen and water. One example of this kind of SCR exhaust posttreatment with gaseous reducing agent is known from International Patent Disclosure WO 99/012105.
In the exhaust gas posttreatment device known from the prior art, an ammonia storage substance, or a mixture of various ammonia storage substances, is present in the storage container, and from these substances, ammonia is released by thermal desorption or thermolysis, or in other words by the effect of temperature. Suitable storage substances can for instance be salts, in particular chlorides or sulfates or one or more alkaline earth elements, such as MgCl2 or CaCl2 and/or one or more 3d side group elements, such as manganese, iron, cobalt, nickel, copper, and/or zinc.
Organic absorbers and ammonium salts, such as ammonium carbamate, are also suitable ammonia storage substances that can be used in the exhaust gas posttreatment device of the invention and the method of the invention. It is definitive for all these substances that the decomposition process is completely reversible. This means nothing else than that after the reservoir has cooled down to the initial temperature level, the initial substances are present again in unchanged form.
In order for the driver of a vehicle equipped with this kind of exhaust gas posttreatment device not to have to refill the storage container himself, the storage capacity of the storage container is designed such that it does not need to be refilled except during scheduled maintenance, such as an inspection.
In practice, a storage volume of approximately 10 liters has proved suitable. A reservoir with a volume has a not inconsiderable thermal capacity.
Since, because of thermal conduction inside the storage container, a virtually constant temperature prevails in the entire storage container, every time the engine is started the storage container, which has meanwhile cooled down, must be reheated again to the operating temperature of 60° C. or 70° C., for example. The resultant energy consumption leads to an increase in fuel consumption of the engine.